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Quantifying uncertainties of sandy shoreline change projections as sea level rises
Sandy shorelines are constantly evolving, threatening frequently human assets such as buildings or transport infrastructure. In these environments, sea-level rise will exacerbate coastal erosion to an amount which remains uncertain. Sandy shoreline change projections inherit the uncertainties of future mean sea-level changes, of vertical ground motions, and of other natural and anthropogenic processes affecting shoreline change variability and trends. Furthermore, the erosive impact of sea-level rise itself can be quantified using two fundamentally different models. Here, we show that this latter source of uncertainty, which has been little quantified so far, can account for 20 to 40% of the variance of shoreline projections by 2100 and beyond. This is demonstrated for four contrasting sandy beaches that are relatively unaffected by human interventions in southwestern France, where a variance-based global sensitivity analysis of shoreline projection uncertainties can be performed owing to previous observations of beach profile and shoreline changes. This means that sustained coastal observations and efforts to develop sea-level rise impact models are needed to understand and eventually reduce uncertainties of shoreline change projections, in order to ultimately support coastal land-use planning and adaptation.
Probabilistic assessment of overtopping of sea dikes with foreshores including infragravity waves and morphological changes : Westkapelle case study
Shallow foreshores in front of coastal dikes can reduce the probability of dike failure due to wave overtopping. A probabilistic model framework is presented, which is capable of including complex hydrodynamics like infragravity waves, and morphological changes of a sandy foreshore during severe storms in the calculations of the probability of dike failure due to wave overtopping. The method is applied to a test case based on the Westkapelle sea defence in The Netherlands, a hybrid defence consisting of a dike with a sandy foreshore. The model framework consists of the process-based hydrological and morphological model XBeach, probabilistic overtopping equations (EurOtop) and the level III fully probabilistic method ADIS. By using the fully probabilistic level III method ADIS, the number of simulations necessary is greatly reduced, which allows for the use of more advanced and detailed hydro- and morphodynamic models. The framework is able to compute the probability of failure with up to 15 stochastic variables and is able to describe feasible physical processes. Furthermore, the framework is completely modular, which means that any model or equation can be plugged into the framework, whenever updated models with improved representation of the physics or increases in computational power become available. The model framework as described in this paper, includes more physical processes and stochastic variables in the determination of the probability of dike failure due to wave overtopping, compared to the currently used methods in The Netherlands. For the here considered case, the complex hydrodynamics like infragravity waves and wave set-up need to be included in the calculations, because they appeared to have a large influence on the probability of failure. Morphological changes of the foreshore during a severe storm appeared to have less influence on the probability of failure for this case. It is recommended to apply the framework to other cases as well, to determine if the effects of complex hydrodynamics as infragravity waves and morphological changes on the probability of sea dike failure due to wave overtopping as found in this paper hold for other cases as well. Furthermore, it is recommended to investigate broader use of the method, e.g., for safety assessment, reliability analysis and design.
A three dimensional semi-analytical model for the prediction of gate vibrations immersed in fluid
A model is developed to predict bending vibrations of flood gates with fluid on both sides. The liquid flow is three-dimensional and the gate is represented as a thin plate. The fluid response is considered within the linear potential flow theory including the effect of compressibility and the generation of free surface waves. This way, the hydrodynamic fluid pressure exerted on the gate is predicted accurately in both low and high-frequency regimes. Both the structural and fluid responses are expressed in the modal domain as a superposition of modes. A semi-analytical solution of the fluid-interaction problem is obtained by describing the complete system in terms of in vacuo gate modes, which is computationally efficient compared to existing numerical methods. This allows for the accurate prediction of flood gate vibrations for a large number of simulations, making it possible to perform fatigue calculations and probabilistic evaluations. The case of a typical flat flood gate subjected to an impulsive wave impact is studied with the developed model. Results show the capability of the model to efficiently quantify flood gate vibrations considering the involved fluid-structure interaction, which can lead to more economical designs compared to common engineering practice.
Hydrodynamic conditioning of diversity and functional traits in subtidal estuarine macrozoobenthic communities
Variations in abundance and diversity of estuarine benthic macrofauna are typically described along the salinity gradient. The influence of gradients in water depth, hydrodynamic energy and sediment properties are less well known. We studied how these variables influence the distribution of subtidal macrofauna in the polyhaline zone of a temperate estuary (Westerschelde, SW Netherlands). Macrofauna density, biomass and species richness, combined in a so-called ecological richness, decreased with current velocities and median grain-size and increased with organic carbon of the sediment, in total explaining 39% of the variation. The macrofauna community composition was less well explained by the three environmental variables (approx. 12e15% in total, with current velocity explaining approx. 8%). Salinity, water depth and distance to the intertidal zone had a very limited effect on both ecological richness and the macrofauna community. The proportion of (surface) deposit feeders (including opportunistic species), decreased relative to that of omnivores and carnivores with increasing current velocity and sediment grain-size. In parallel, the proportion of burrowing sessile benthic species decreased relative to that of mobile benthic species that are able to swim. Correspondingly, spatial variations in hydrodynamics yielded distinct hotspots and coldspots in ecological richness. The findings highlight the importance of local hydrodynamic conditions for estuarine restoration and conservation. The study provides a tool based on a hydrodynamic model to assess and predict ecological richness in estuaries.
Damage characterization of rock slopes
In order to design reliable coastal structures, for present and future scenarios, universal and precise damage assessment methods are required. This study addresses this need, and presents improved damage characterization methods for coastal structures with rock armored slopes. The data used in this study were obtained from a test campaign carried out at Deltares within the European Union (EU) Hydralab+ framework. During these tests, advanced measuring techniques (digital stereo photography) were used, which are able to survey the full extension of the structure and identify local variations of damage. The damage characterization method proposed here is based on three fundamental aspects: clear damage concepts, precise damage parameters, and high resolution measuring techniques. Regarding damage concepts, first, the importance of the characterization width is studied. For damage parameters obtained from the maximum erosion depth observed in a given width (E3D,m), the measured damage increases continuously with increased characterization width. However, for damage parameters obtained from width-averaged profiles (S and E2D), the measured damage reduces with increased characterization width. Second, a new definition of damage limits (damage initiation, intermediate damage, and failure) is presented and calibrated. Regarding the damage parameters, the parameter E3D,5, which describes the maximum erosion depth within the characterization width, is recommended as a robust damage parameter for conventional and non-conventional configurations based on four main characteristics: its low bias, its low random error, the ability to distinguish damage levels, and its validity and suitability for all types of structures (conventional and non-conventional). In addition, the results from this study show that the damage measured with the damage parameter E3D,5 presents an extreme value distribution.
Climate change-driven losses in ecosystem services of coastal wetlands : a case study in the West coast of Bangladesh
Climate change is globally recognized as one of the key drivers of degradation of coastal wetland ecosystems, causing considerable alteration of services provided by these habitats. Quantifying the physical impacts of climate change on these services is therefore of utmost importance. Yet, practical work in this field is fragmented and scarce in current literature, especially in developing countries which are likely to suffer most from the adverse climate change impacts. Using a coherent scenario-based approach that combines assessment of physical impacts with economic valuation techniques, here we quantify potential climate change driven losses in the value of wetland ecosystems services due to relative sea-level rise (RSLR)-induced inundation in the vulnerable Western coastal area of Bangladesh in 2100. The results show a small inundation area in 2100 under the three IPCC climate scenarios of RCP2.6 (with 0.25m of RSLR), RCP6.0 (with 1.18m of RSLR), and RCP8.5 (with 1.77m of RSLR) for the coastal wetland ecosystems including the Sundarbans mangrove forest, neritic system and aquaculture ponds. In all scenarios, RSLR will drive a loss in the total value of ecosystem services such as provision of raw materials, and food provision, ranging from US$ 0–1 million to US$ 16.5–20 million, respectively. The outcomes of this study reveal that RSLR-induced inundation on its own, is unlikely to be a major threat to the wetland ecosystems in Western coast of Bangladesh. This would suggest that other climate change impacts such as coastal erosion, increase in frequency of cyclone events, and sea temperature rise might be the likely primary drivers of change in the value of wetland ecosystems services in this area.
An integrated and interactive toolbox for the design of coastal infrastructure
In order to support clients and stakeholders in the early design phases of interventions in coastal systems, the Coastal Design and Support (CoDeS) tools framework is developed. This framework combines relatively simple tools and empirical relations to assess various aspects of designs and their impacts. The tools are linked to a consistent and easy-to-use Graphical User Interface (GUI) which is applicable around the globe.
How tides and waves enhance aeolian sediment transport at the Sand Motor mega nourishment
In this paper we present a two-dimensional application of the Windsurf modeling framework on the Sand Motor mega-nourishment in The Netherlands that allows for detailed simulation of the interaction between subtidal and subaerial processes.
Undular bore development over a laboratory fringing reef
Several studies have reported the development of undular bores over fringing coral reefs but the importance of this phenomenon for reef hydrodynamics has never been studied. In this study, we investigate undular bore development over reef-type profiles based on a series of laboratory experiments. More specifically, we aim to characterize the conditions under which undular bores develop, and analyse how their development affect the hydrodynamics at the toe of the reef-lined beach and the resulting wave run-up.
Diffusion of a mega feeder nourishment : assessing 5 years of sand engine spreading
Feeder nourishments, where sand quantities of O (10 million m3) are placed locally to feed adjacent coastal stretches, are suggested nowadays as an alternative for local, smaller-scale nourishments (< 1 million m3). The Sand Engine project that is implemented in the Netherlands in 2011 consists of 21.5 million m3 of nourished sediment, and is the largest existing feeder nourishment. In this paper the morphological development of the Sand Engine mega feeder nourishment and the adjacent coastal sections is presented. The alongshore extent of the analysis is 17 km and spans a coastal cell between 2 harbor entrances.